Field of the Invention: This invention relates to a method for the manufacture of a continuous sheet of flat glass by supporting molten glass on a pool of molten metal while forming and cooling glass. More particularly, this invention relates to a method for controllably cooling the glass as it advances along the molten metal during forming.
Description of the Prior Art: Molten glass may be delivered onto molten metal and formed into a continuous sheet or ribbon of glass according to the teachings of Heal, U.S. Pat. No. 710,357 or of Hitchcock, U.S. Pat. No. 789,911 or according to the patents of Pilkington, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816. These patents describe processes in which molten glass delivery techniques vary. In one practice disclosed by Pilkington, molten glass is delivered through a long, narrow canal and over a lip from which the molten glass falls onto molten metal and spreads laterally outwardly on the molten metal for forming.
In the methods of Heal and Hitchcock, molten glass is delivered over a refractory bridge or wall onto molten metal. In the method of Heal the glass flows downwardly from a supporting bridge onto the surface of a pool of molten metal. In the method of Hitchcock, the molten glass is delivered substantially horizontally through an opening in a wall onto the surface of a pool of molten metal which is maintained at substantially the same elevation as the bottom of the opening in the wall over which the molten glass is delivered.
Following delivery of molten glass onto molten metal in the processes described above, the glass is advanced along the surface of the pool of molten metal as a layer and is cooled to form a dimensionally stable, continuous sheet or ribbon of glass. This is so in all of the described methods. In the methods of Hitchcock and Heal, the continuous sheet of glass is removed horizontally from the pool of molten metal and is thereafter conveyed away from the apparatus which is formed. In the process described by Hitchcock, an annealing chamber is provided so that the glass may be annealed while supported on a shallow segmented pool of molten metal. In the processes of Pilkington, the glass is removed from the pool of molten metal by lifting it slightly and supporting it on rolls to convey it away from the apparatus containing the pool of molten metal on which the glass is formed.
In the patent to Hitchcock there is shown a plurality of barriers for segregating the bottom of a forming chamber into a series of compartments for containing different metal alloys having different melting points. The compartments are provided with heating elements submerged in the molten metal for controlling the temperature of the molten metal in each compartment and maintaining it in a molten state. Barriers are also illustrated in U.S. Pat. No. 3,241,939 to Michalek and in U.S. Pat. No. 3,485,617 to Lawrenson. These barriers are disclosed as being useful to retard the flow of molten metal along or counter to the direction of glass advance on the surface of a pool of molten metal that is presumably of like composition throughout. Temperature regulators are illustrated in the bottom of the chamber disclosed by Michalek and in the molten metal in the chamber disclosed by Lawrenson.
In the practice of this invention, as in known forming methods, a continuous sheet of flat glass is produced by a method including the following steps: molten glass is delivered onto the surface of a pool of molten metal maintained within a forming chamber. The molten metal is preferably tin, an alloy of tin or some other metal having a specific gravity greater than glass, having a melting point lower than the glass to be formed, and being substantially non-reactive with the glass at its melting temperature. The molten glass is advanced along the surface of the pool of molten metal and is cooled during such advance to form a dimensionally stable, continuous sheet of glass.
Forces are applied to the glass while it is advanced along the surface of the pool of molten metal. There are forces applied to the glass which are aligned substantially along its path and which cause it to be advanced in that path and to be attenuated or thinned during its advance. These forces, characterized as longitudinal tractive forces, may be applied to the glass at any location along its path. They are preferably applied to the glass at locations well along its path of advance and are transferred to the hotter, more fluid, glass primarily by the surface tension of the glass.
After the glass has been cooled sufficiently to become dimensionally stable (that is, it has reached its final width and thickness) it is advanced farther along the surface of the molten metal a short distance and then is lifted from the surface of the pool of molten metal. It may be lifted and conveyed upwardly, slightly upwardly then horizontally, or horizontally from the pool of molten metal by applying a tractive force to the sheet of glass. Such a tractive force is preferably applied at a location sufficiently remote from the pool of molten metal so that the means employed to apply such a force to the glass does not impart a permanent marking or imprint to the surface of the glass. In the practice of this invention, this force may be effectively transferred along the glass through the location of glass removal or lifting and to hotter, more fluid, glass in the upstream region of the forming chamber so as to cause attenuation of the glass. In a preferred embodiment the glass is conveyed upwardly along a path that forms an angle of at least about 60.degree. with the horizontal surface of the pool of molten metal upon which the molten glass has been formed into a continuous sheet. More preferably, the glass is lifted and conveyed upwardly along a substantially vertical path that is from 80.degree. to 100.degree. from the horizontal path of the advancing glass prior to lifting.
In the practice of this invention, it is preferred that a layer of molten glass be delivered along a substantially horizontal path onto the pool of molten metal with the upper surface of the pool of molten metal upon which the glass is supported and maintained at substantially the same elevation as such delivery. Nevertheless, this invention may be successfully practiced to improve methods wherein glass is delivered for forming by pouring as in U.S. Pat. No. 3,220,816 or by delivering a preformed ribbon as in U.S. Pat. No. 3,083,551.